1. Field of the Invention
This invention relates to a process for forming a gas mixture containing hydrogen and nitrogen gas for ammonia synthesis, wherein a feed gas mixture containing hydrogen and adsorbable impurities is separated by adiabatic pressure swing adsorption.
2. Description of the Prior Art
In the general practice of forming gas mixtures containing hydrogen and nitrogen for ammonia synthesis, the hydrogen constituent of the synthesis gas mixture may suitably be produced in any of a variety of process steps, such as steam reforming of natural gas or naphtha feedstocks, partial oxidation of hydrocarbon feedstocks, or gasification of coal. Regardless of the type of hydrogen formation method employed, the hydrogen-bearing stream derived from such formation step will typically contain a number of impurities, such as carbon dioxide, carbon monoxide, methane and water. Accordingly the hydrogen-rich gas mixture from the hydrogen-forming step is generally subjected to further treatment steps such as carbon monoxide shift conversion for removal of the carbon monoxide content of the mixture, carbon dioxide removal by selective absorption of carbon dioxide from the gas mixture by a suitable liquid solvent in a wash column, and final purification of the gas mixture by selective adsorption for removal of residual impurities therefrom to yield high purity hydrogen make-up gas. Thereafter the high purity hydrogen gas may be blended with compressed nitrogen from an external source to form the ammonia synthesis gas mixture containing hydrogen and nitrogen in the desired proportion, as for example a stoichiometric 3:1 molar ratio of hydrogen to nitrogen.
In the final purification step in the above-described process, wherein residual impurities are removed from the hydrogen containing make-up gas stream by selective adsorption, adiabatic pressure swing adsorption processes may suitably be employed such as those disclosed in Wagner U.S. Pat. No. 3,430,418 or Fuderer et al. U.S. Pat. No. 3,986,849. The Wagner process employs four adsorbent beds and is capable of yielding 99.9999% hydrogen product gas with no detectable quantities of the aforementioned impurities and at recovery levels on the order of 75-80%. The Fuderer et al process employs at least seven adsorbent beds of which at least two receive feed gas during the process cycle, with at least three pressure equalization stages in the cycle. The Fuderer process is capable of yielding 99.9999% hydrogen product gas at hydrogen recovery levels on the order of 85-90%.
In the above-described adiabatic pressure swing adsorption processes, even though extremely high purity hydrogen product is obtained, a significant amount of the hydrogen contained in the feed gas mixture to the pressure swing process is lost in the waste gas, i.e., countercurrent depressurization gas and purge gas, discharged from the process. Another disadvantage of the prior art pressure swing adsorption processes in such ammonia synthesis gas applications is that they require a relatively high inventory of adsorbent in the adsorbent beds of the process system.
With regard to the overall ammonia synthesis gas production process, using the previously described process steps to produce a high-purity hydrogen gas stream as makeup for the synthesis gas mixture, the nitrogen gas which is blended with the high-purity hydrogen gas to yield the final synthetis gas mixture is typically available from external source means, such as cryogenic or pressure swing adsorption air separation plants, liquid nitrogen vaporization systems and nitrogen gas pipelines, at relatively low pressures, on the order of for example 10-100 psia. Inasmuch as the required pressure of the product synthesis gas mixture is generally higher than 400 psia, substantial compression energy must be expended to compress the low pressure nitrogen to the high pressure levels necessary for formation of the product gas mixture.
Accordingly, it is an object of this invention an improved process for forming a gas mixture containing hydrogen and nitrogen for ammonia synthesis.
It is another object of the invention to provide a process for production of ammonia synthesis gas mixtures which is characterized by higher percent recovery of hydrogen in the final purification pressure swing adsorption step than is realized with prior art processes.
It is another object of this invention to provide a process for producing a synthesis gas mixture for ammonia synthesis which requires less adsorbent material in the pressure swing adsorption step than is required by prior art systems.
It is still another object of this invention to provide a process for producing a synthesis gas mixture of hydrogen and nitrogen wherein the external compression requirement for compressing low pressure make-up nitrogen gas to final product pressure is substantially reduced relative to prior art processes.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.